1. Field of the Invention
The present invention relates to an AC light emitting diode (LED), and more particularly, to an AC LED having an improved transparent electrode structure.
2. Discussion of the Background
GaN based light emitting diodes (LEDs) have been applied and developed for about 10 years. The GaN based LEDs have considerably changed LED technologies and currently are used for a variety of applications, such as full color LED displays, LED traffic signals and white LEDs. Recently, high-efficiency white LEDs are expected to replace fluorescent lamps. Particularly, the efficiency of the white LED reaches a level similar to that of general fluorescent lamps.
In general, an LED emits light by forward current and requires the supply of DC. Therefore, when the LED is used by being directly connected to an AC power source, the LED is repeatedly turned on/off depending on the direction of the current. As a result, there is a problem in that the LED does not continuously emit light, and may be easily damaged due to reverse current.
In order to solve such a problem of the LED, an LED that can be used by being directly connected to a high-voltage AC power source has been disclosed in PCT Publication No. WO 2004/023568(A1), entitled “LIGHT-EMITTING DEVICE HAVING LIGHT-EMITTING ELEMENTS” by SAKAI et al.
FIG. 1 is a partial plan view illustrating the AC LED according to PCT Publication No. WO 2004/023568(A1), and FIG. 2 is a sectional view taken along line IV-IV in FIG. 1.
Referring to FIGS. 1 and 2, a plurality of light emitting cells 1 are formed on a substrate 10. Further, a p-electrode 22 and n-electrode 24 are provided on each of the light emitting cells 1. Air-bridge wires 28 electrically connect the p- and n-electrodes 22 and 24 of the light emitting cells adjacent to each other, so that the plurality of light emitting cells 1 are connected in series.
Each of the light emitting cells comprises n-GaN layer 14 and p-GaN layer 20. Although not shown, an InGaN active layer may be interposed between the n-GaN and p-GaN layers 14 and 20. The p-electrode 22 is formed on the p-GaN layer 20, and the n-electrode 24 is formed on the n-GaN layer 14. Meanwhile, the air-bridge wires 28 electrically connect the n- and p-electrodes 22 and 24.
According to PCT Publication No. WO2004/023568(A1), LEDs (light emitting cells) are two-dimensionally connected in series on an insulative substrate such as a sapphire substrate to form an LED array. Such two LED arrays are connected in reverse parallel on the sapphire substrate. As a result, a single chip light emitting device that can be driven by means of an AC power supply is provided.
However, when the p- and n-electrodes 22 and 24 are connected through the air-bridge wires 28 as shown in FIG. 1, the forward current flowing through the air-bridge wires 28 is locally concentrated on a region of each of the p-electrodes 22. Accordingly, the current density is increased in the region of each of the p-electrodes 22 and the current density in the other region thereof is relatively lowered. The local increase of the current density can not only limit a light emitting region of an active layer but also cause a peeling phenomenon of the p-electrode 22 due to the excessive current density. Further, the local increase of the current density increases a junction temperature in its region, thus reducing the light emitting efficiency. In a case where current is increased in order to obtain high output power, such problems become worse. Therefore, there is required an LED in which the current is prevented from being locally concentrated on the p-electrodes 22 and is uniformly dispersed in the entire p-electrodes.